MICRO-ENGINEERED CATHODE INTERFACE STUDIES

Transcription

1 MCROENGNEERED CATHODE NTERFACE STUDES Rajiv Doshi, Timothy Kueper, Zolton Nagy and Michael Krumpelt Electrochemical Technology Program Chemical Technology Division Argonne National Laboratory 97 S. Cass Avenue Argonne, L 6439 f h e submitted manuscript has been created by the University o? Chicago as Operator of Argonne National Laboratory ( Argonne ) under Contract No. W3119ENG38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paidup, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the 1 Government. August For presentation at the Fuel Cells 97 Review Meeting, Morgantown, West Virginia, August 2628,1997.

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed,or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation,or favoring by the United Stata Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

3 MCROENGNEERED CATHODE NTERFACE STUDES NTRODUCTON Rajiv Doshi ) Michael Krumpelt ) Electrochemical Technology Program Chemical Technology Division, B25 Argonne National Laboratory 97 South Cass Avenue Argonne, L 6439 The aim of this work is to increase the performance of the cathode in solid oxide fuel cells (SOFCs) operating at 1 C by decreasing the polarization resistance from.2 Qcm2 at 3 malcm2. Decreased polarization resistance will allow operation at higher current densities. This work is in support of the Westinghouse tubular SOFC technology using YSZ electrolyte and strontium doped lanthanum manganite (LSM) cathode. THE PROBLEM As a result of work performed last year at Argonne National Laboratory and information derived from the literature, the limitations at the cathode/electrolyte interface can be classified into two main areas. First, the ionic conductivity of the LSM cathode material is low which limits the reaction zone to an area very close to the interface, while the rest of the cathode thickness acts essentially as current collector with channels for gas access. Second, the electronic conductivity in YSZ is very low which limits the reaction zone to areas that are the boundaries between LSM and YSZ rather than the YSZ surface away from LSM at the interface. APPROACH Possible solutions to this problem being pursued are: 1) introducing an ionic conducting YSZ phase in LSM to form a porous twophase mixture of LSM and YSZ; 2) applying a thin interlayer between the electrolyte and the cathode where the interlayer has high ionic and electronic conductivity and high catalytic activity for reduction of,; 3) increasing the ionic conductivity in the LSM by suitable doping; and 4) increasing the electronic conductivity in the electrolyte by doping or by depositing an appropriate mixed conducting layer on the YSZ before applying the cathode. PROCEDURE We obtained YSZ electrolyte discs of 2 pm from Marketech nternational. The appropriate microstructure of the electrode to be studied was then created on top of the YSZ 1.

4 membrane so as to form the working electrode. A reference electrode was also made the same way on the same side of the disk as the working electrode but separated from the working electrode by about 5 mm. Similarly, a counter electrode and another reference electrode were deposited on the other side of the YSZ disc. The experimental arrangement is shown in Fig. 1. Counter electrod Ref, Reference electrode Fig. 1. Electrode setup for polarization tests. Polarization resistance of each electrode was measured against the corresponding reference electrode using impedance spectroscopy. Overpotential measurements were performed as a function of current density using a galvanostat, and the potential of the working and counter electrodes were measured against each other and against the corresponding reference electrodes. RESULTS The effect of low ionic conductivity in LSM on the cathode polarization resistance (R,) is evident from Fig. 2, which shows a dense LSM disc bonded to a YSZ disc with a thin layer of LSM applied between the two. These edges were sealed as shown with a glassplus cement seal so that gas access to any LSWYSZ interface is blocked off. n this way, all the oxygen from the air is forced to difise through the dense LSM. The R, from impedance spectroscopy in such a case was high, 11 kqcm*. A cathode with high oxide ion conductivity should decrease the polarization resistance. dense L%$ro.,Mn3 (2OOp.m) r glasdcement seal YSZ Disc (2 pm) Fig. 2. Configuration of dense LSM on YSZ with sealed edges. 2.

5 n another experiment a YSZ disc was ion implanted with manganese ions to.1 pm below the surface. Upon heating up to 9"C, the polarization resistance was found to be low,.38 Qcm2. Over time, this value increased as manganese ions diffused away from the surfaceofthe YSZ into the bulk. However, it showed that increasing the ionic conductivity of YSZ at the surface helps to spread out the reaction zone and decreases polarization. Based on the above two observations, compositions were devised to increase the ionic conductivity of the cathode by doping an LSM composition with Co and by increasing the substitution of La by Sr. New materials like the cobaltferrite composition were also tried. The aim here is to either replace the LSM cathode or use the new materials as a thin interlayer between LSM and YSZ. A cobaltdoped LSM composition, (L~,6Sro,4)o,9Mno~95Coo,53 (designated LSCM2) sintered at 13 C and tested last month showed good performance (R, around.1 Qcm2) as a cathode on YSZ with no load. This was presumably due to increased ionic conductivity from the cobalt doping. Further testing at different loads (current densities) showed good performance as well. Figure 3 is a plot of overpotential versus current density after holding the sample at a 1 C for a cumulative 48 hours with two temperature cycles where it was cooled to room temperature and heated back up to 1 C. The overpotential at 3 ma/cm2 was 39 mv, compared with a typical 6 mv overpotential for Westinghouse's cathode at the same current density E.a X + u F ~ ~ ~ o. ~ ~ ~ o. ~ ~ o. Q ~ " o. Q, Cysz ~o.oso, T = OOO'C No. of Temperature Cycles = 2 No. of Hours at 1" before measurement= Current Density (malcm2) Fig. 3. Overpotential of LSCM2 decreases with increasing. Another cobaltdoped composition, (Lao,ssSro, 5),9Mno,98C,23 (LSCMl), was tested both with sintering at 13 C and sintered in situ at the operating temperature of 1 C, (see Fig. 4). The sintered composition exhibited a higher R, of about.34 Qcm2, while the in situ sintered sample had an initial R, of.3 Qcm2 without any load. This compares very well to a value of.2 Qcm2 for Westinghouse's cathode at 3 ma/cm2. The in situ composition started 3.

6 sintering with time at 1 C, which caused an increase of the R, from an initial value of.3 Qcm to.1 Qcm after 67 hr. Sintering this composition caused a decrease in the actual area of contact between YSZ and LSCM while increasing the area of contact between LSCM particles. This indicated that cobaltdoped LSM (LSCM) shows promise as an interlayer if it is made with a large contact area between LSCM and YSZ that does not decrease with time under operating conditions..35 7) ! (La.85Sr.~5).9Mn.98c.23 sintered at 13 C sintered in situ Time (hrs) Fig. 4. LSCM1 shows good performance initially when sintered in situ. t has been reported in the literature that manganites of praseodymium (PrMnO,) showed higher performance than LaMnO;. To utilize this fact, we synthesized praseodymium counterparts of LSCM 1 and LSCM2, by the Pechini method, (Pro&3r,, 5 ), 9 M n, 9 ~ C ~ (PSCM1), ~ O ~ and (Pro,6Sro,4)o,9~~95C~o,oj3 (PSCM2). t is expected that the cobaltdoping in PSM also increases ionic conductivity. Figure 5 gives the performance of PSCM1 for one electrode sintered at 13 C and another in situ. n this case the 13 C sintered composition exhibited better performance than the one sintered in situ, and both cathodes showed a deterioration in performance with time at 1 C. The PSCM2 also showed the same trend, but with a much steeper increase in R, with time. Since both compositions show the same trend, the deterioration is probably due to change in cathode composition with time, possibly by Co or Mn diffusion from PSCM to YSZ. 4.

7 ::: / sintered in situ sintered at 13OO'C BO Time (hrs) Fig. 5. R, for PSCM1 sharply increases with time after 4 hrs. High ionic and electronic conductivity is well known in L~,6Sro,,Co,,,Feo,83 (LSCF) and similar cobaltite or cobaltferrite compositions. We previously reported data on some LSCF compositions where the LSCF composition was slightly deficient on the Asite, (La,~6Sr,~,),,,,Coo~2Feo~83. While this yielded very low R yalues of.2 Qcm2 or even less initially, the formation of an insulating CoFe,O, caused the R, to increase to almost 1 Qcm2in less than three days. This time we report results on stoichiometric LSCF with the composition La, 6Sr,Co,,Fe,,,,. The polarization resistance, R,, for this LSCF still increases with time, (see Fig: 6 ),'but at a slower rate than before. The ohmic resistance component through the electrolyte plus cathode, if any, also rises. This indicates that the increase in R, is accompanied by an increase in resistance. The rate of increase in resistance indicates the formation of an insulating phase. Both La2Zr2, and SrZrO, have been reported to form when LSCF electrode is used on a YSZ electrolyte at 1 C a,. " 2 2 R, Time (hrs) Fig. 6. Both the R, and the resistance part of ir drop increase with time for LSCF. 5.

8 n order to decrease the interaction between YSZ and LSCF, an attempt was made to deposit on YSZ, followed by the deposition of LSCF on top a thin (< 1 pm) dense layer of Ceo.9Gdo,126 of the ceria layer. The ceria layer made by us turned out to be porous. Nevertheless, twosuch electrodes were tested with the following configurations: stoichiometric LSCF/porous CGlNSZ and LSCM2/porous CGlNSZ. Results of cathode tests of both configurations are shown in Fig. 7. The LSCF cathode showed a higher initial R, (.2 Qcm2)than the previously tested LSCF/YSZ configurations and also showed an increase in R, to.33 Qcm2 over 115 hr. However, the plot shows that the R,value stabilized and, after 115 hr, is less than the LSCFNSZ polarization resistance of 1 Qcm2after 63 hr. Therefore, a ceria interlayer did prevent significant reaction between LSCF and YSZ. Similarly, for the LSCM2 cathode with ceria interlayer, the value of was initially.47 Qcm2, which actually decreased and stabilized at.44 Qcm2 after 115 hr. The reason for a decrease in R, for LSCM2 versus an increase for LSCF is not known at this time; however, it may be linked to differences in thermal expansion coefficient between the cathode and the interlayer / Fig. 7. Stable but higher polarization resistances obtained with an interlayer of porous ceria. n order to improve the overall Rc value for LSCF/CGl/YSZ cathodes, a thin.5 pm dense CG1 film was deposited by Prof. Harlan Anderson at the University of MissouriRolla on a YSZ disc supplied by us. We then applied the LSCF cathode and tested the cell as a function of time. The results of this test (Fig. 8) show a low initial R, of.24 Qcm2which then decreases slightly and monotonously to less than.1 8 Qcm2 over 72 hrs. This electrode again is a combination of steps 3 and 4 as outlined earlier. 6.

9 1.2 2 h N Y b.'5.w a a () Oi. O n RC c v u 2 4 c " 6 8 Time (hrs) Fig. 8. Very low and stable R, obtained with a thin dense interlayer of ceria. We are also currently working on a composition that can be used directly as a cathode with no interlayers. The goal here is to increase the ionic conductivity without decreasing the electronic conductivity of LSM or its catalytic activity for oxygen reduction. n addition, the thermal expansion coefficient will be matched to the YSZ electrolyte and it will not be reactive with YSZ. nitial results show some promise with about a 112 mv polarization at 3 ma/cm2. Further work is in progress to understand and improve this composition. BENEFTS ncreased cathode performance or decreased cathode polarization resistance will help to operate at higher power density or with higher efficiency. FUTUPX WORK Development of new compositions that can be used as cathodes without interlayers. These materials will be analyzed for performance over time and as a function of compositional changes. The aim here is to increase ionic conductivity substantially compared to LSM. Continued development of Co or other transition metaldoped LSM. We will select a promising material and develop a cathode with an interlayer microstructure of a twophase mixture of YSZ and doped LSM. t will be analyzed in terms of performance over time and its thermal expansion coefficient will be measured. 3) Analysis of previously tested cathode/electrolyte and cathode/interlayer/electrolytesamples. These samples will be analyzed for microstructural characteristics including porosity, interface bonding, and interdiffusion of cations across the interface. 7.

10 ACKNOWLEDGEMENT Research sponsored by the Department of Energy through the Morgantown Energy Technology Center under contract no. W3 11 9ENG38 with FETC Project Manager, Richard Johnson; Contractor Project Manager, Kevin M. Myles; Principle nvestigator, Michael Krumpelt. This contract covers work done in FY

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